Purification of gases containing sulfur compounds

ABSTRACT

A system includes a purification unit configured to process a vapor stream including sulfur dioxide. The purification unit includes an inlet configured to allow the vapor stream to enter the purification unit. The purification unit includes a steam coil configured to circulate steam and provide a source of heat. The purification unit includes a packed bed. The purification unit includes a tray configured to accumulate sulfur. The purification unit includes an absorber section configured to remove at least a portion of the sulfur dioxide from the vapor stream. The purification unit includes an outlet configured to allow an effluent with a lower sulfur dioxide content than the vapor stream to exit the purification unit. The system includes a sulfur tank including a vent line in fluid communication with the inlet. The vent line is configured to allow vapor to flow from the sulfur tank to the purification unit.

CLAIM OF PRIORITY

This application is a continuation of and claims the benefit of priorityto U.S. patent application Ser. No. 16/413,367, filed on May 15, 2019,which claims the benefit of U.S. Provisional Application Ser. No.62/672,301, filed on May 16, 2018, the entire contents of which ishereby incorporated by reference.

TECHNICAL FIELD

This specification relates to operating industrial facilities, forexample, a refinery or other industrial facilities that process crudeoil and produce sulfur as a byproduct.

BACKGROUND

After raw hydrocarbons are extracted from a reservoir, the hydrocarbonscan be refined to produce commercial fuels and other products.Hydrocarbon refining processes are chemical engineering processes usedin refineries to transform raw hydrocarbons into the various products,for example, liquefied petroleum gas (LPG), gasoline, kerosene, jetfuel, diesel oils, fuel oils, and other products. Refineries are largeindustrial complexes that involve many different processing units andauxiliary facilities, for example, utility units, storage tanks, andother auxiliary facilities. Each refinery can have its own uniquearrangement and combination of refining processes determined, forexample, by the refinery location, desired products, economicconsiderations, or other factors.

Hydrocarbons extracted from a reservoir can contain various impurities.Hydrocarbons that are contaminated with significant amounts of sulfurcompounds, such as hydrogen sulfide, is considered sour, whilehydrocarbons that are contaminated with little or negligible amounts ofsulfur compounds is considered sweet. Hydrogen sulfide, in particular,is highly poisonous, corrosive, and flammable. Therefore, the presenceand handling of hydrogen sulfide is not only an operational concern(with respect to equipment and piping corrosion) but also a safetyconcern. The hydrocarbon refining processes can include processes thatremove such impurities from the raw hydrocarbons, for example, beforethe hydrocarbons are transformed into the various products mentionedpreviously. Many refineries also include sulfur recovery processes thatconvert hydrogen sulfide (removed from the hydrocarbons) into elementalsulfur, which can be stored and subsequently sold on the market.

SUMMARY

The present disclosure describes technologies relating to purifyinggases that contain sulfur compounds.

Certain aspects of the subject matter described here can be implementedas a system including a purification unit configured to process a vaporstream including sulfur dioxide. The purification unit includes an inletat or near a bottom of the purification unit. The inlet is configured toallow the vapor stream to enter the purification unit. The purificationunit includes a steam coil configured to circulate steam and provide asource of heat. The purification unit includes a packed bed. Thepurification unit includes a tray above the packed bed, and the tray isconfigured to accumulate sulfur. The purification unit includes anabsorber section above the tray, and the absorber section is configuredto remove at least a portion of the sulfur dioxide from the vaporstream. The purification unit includes an outlet at or near a top of thepurification unit. The outlet is configured to allow an effluent with alower sulfur dioxide content than the vapor stream to exit thepurification unit. The system includes a sulfur tank including a ventline in fluid communication with the inlet. The vent line is configuredto allow vapor to flow from the sulfur tank to the purification unit.

This, and other aspects, can include one or more of the followingfeatures. The packed bed can include structured packing, random packing,or combinations of both. The packed bed can include a wire mesh andrings. Each ring can have a length and a diameter, and the length can beapproximately equal to the diameter. The absorber section can include abed of catalyst having a honeycomb structure. The steam coil can beconfigured to maintain a temperature within the purification unit atapproximately 180 degrees Celsius (° C.). The purification unit caninclude a sulfur drain configured to allow accumulated sulfur flow fromthe tray to exit the purification unit. The system can include electrictracing, steam tracing, or combinations of both to maintain in the ventline and the sulfur drain, a sufficiently high temperature to preventsolidification of sulfur.

The details of one or more implementations of the subject matter of thisspecification are set forth in the accompanying drawings and thedescription. Other features, aspects, and advantages of the subjectmatter will become apparent from the description, the drawings, and theclaims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram of an example sulfur fume purificationsystem.

FIG. 2 is a schematic diagram of an example purification unit, which isa part of the sulfur fume purification system of FIG. 1 .

FIG. 3 is a cross-sectional view of an example absorber section, whichis a part of the purification unit of FIG. 2 .

DETAILED DESCRIPTION

Molten sulfur (that is, sulfur in liquid form) can contain compoundssuch as hydrogen sulfide (H₂S) and sulfur dioxide (SO₂). H₂S isflammable, and both H₂S and SO₂ are toxic and can potentially acceleratecorrosion of various materials in the presence of water. Molten sulfurstorage tanks that vent directly to the atmosphere contribute to theoperating plant emissions and can additionally expose personnel to theseharmful components. Reducing emissions of such sulfur containingcompounds is desirable, especially as emissions standards across theworld become increasingly stringent. This specification describes amethod for safely diverting a molten sulfur storage tank vent to asulfur recovery unit in order to lower SO₂ emissions and increase sulfurrecovery. The subject matter described in this specification can beimplemented in particular implementations, so as to realize one or moreof the following advantages. By venting directly to a sulfur recoveryunit, sulfur containing compounds and heat can be removed from the ventgas before being expelled to the atmosphere. The sulfur recovery processcleans the vent gas before release to the atmosphere and effectivelylowers the amount of emissions and atmospheric pollution. Therefore,risk of exposing personnel to harmful sulfur-containing compounds, suchas H₂S or SO₂, can be reduced.

FIG. 1 shows an example sulfur fume purification system 100. The sulfurfume purification system 100 includes a sulfur tank 110, a purificationunit 200, and a vent line 112 connecting the sulfur tank 110 to thepurification unit 200. The sulfur tank 110 can hold a volume of sulfurin liquid form (also referred as molten sulfur). The sulfur tank 110 caninclude a heat source to maintain a temperature sufficiently high tokeep the sulfur in the tank 110 in liquid form (that is, prevent thesulfur in the tank 110 from solidifying). The heat source can be, forexample, a steam coil submerged in the molten sulfur inside the tank110, an external steam jacket lining the wall of the tank 110, or acombination of these. For example, the heat source can maintain atemperature of approximately 150 degrees Celsius (° C.) to approximately160° C. within the tank 110. In this specification, “approximately”means a deviation or allowance of up to 10 percent (%) and any variationfrom a mentioned value is within the tolerance limits of any machineryused to manufacture the part. The sulfur tank 110 can also includeinsulation to mitigate or prevent heat loss from the tank 110 to thesurrounding environment. The sulfur tank 110 can operate at a pressureslightly higher than atmospheric pressure, such that vapor can flow fromthe sulfur tank 110, through the purification unit 200, and out to theatmosphere. The operating pressure of the vapor space of the sulfur tank110 can be the vapor pressure of the molten sulfur in the tank 110. Insome implementations, the sulfur tank 110 normally operates at apressure in a range between atmospheric pressure and 0.5 pounds persquare inch gauge (psig).

In the presence of oxygen, the sulfur in the tank 110 can expel sulfurdioxide into the vapor space of the tank 110. In the case where hydrogensulfide is dissolved in the liquid sulfur, hydrogen sulfide can alsoescape from the sulfur into the vapor space of the tank 110. The ventline 112 can be connected to an outlet nozzle located on the roof of thetank 110. Vapor from the tank 110 can exit through the vent line 112 andflow to the purification unit 200. The vent line 112 can include heattracing, such as electric tracing, steam tracing, or a combination ofboth to maintain a temperature within the vent line 112 that issufficiently high to prevent solidification of sulfur. For example, theheat tracing can maintain a temperature of approximately 150° C. toapproximately 160° C. within the vent line 112.

FIG. 2 shows an example purification unit 200, which is part of thesulfur fume purification system 100 shown in FIG. 1 . The purificationunit 200 includes an inlet 202, a steam coil 204, a packed bed 206, atray 208, an absorber section 210, and an outlet 212. The inlet 202 canbe connected to the vent line 112, such that vapor can flow from thetank 110 and enter the purification unit 200. The purification unit 200can also include a drain (214 and 224), a manhole 222, and a pressuresafety valve (PSV, also referred as a relief valve) 226. In someimplementations, the purification unit 200 includes multiple steam coils(204A, 204B). One steam coil (204A) can be located near the bottom ofthe purification unit 200, and another steam coil can (204B) can belocated near the middle of the purification unit 200. Steam can becirculated through the steam coils 204A and 204B to provide a source ofheat to the purification unit 200. Under operation, the steam coils(204A and 204B) can maintain a temperature within the purification unit200 that is sufficiently high to prevent solidification of sulfur. Theoperating temperature of the purification unit 200 can be higher thanthe operating temperature of the tank 110 to account for heat losses.For example, under operation of the steam coils 204A and 204B, thepurification unit 200 can operate at a temperature of approximately 180°C. Like the tank 110 shown in FIG. 1 , the purification unit 200 canoptionally include external steam jacketing, insulation, or acombination of both to maintain the desired temperature and to preventor mitigate heat loss from the purification unit 200. Heat insulationcan also be provided on the vent line 112 and any or all sections of thepurification unit 200.

The packed bed 206 can include random packing, such as glass Raschigrings or other packing material. Raschig rings are pieces of tube thatare approximately equal in length and diameter. The packed bed 206 canalso include structured packing, such as wired mesh packing or otherstructured packing. The packed bed 206 can be located above the steamcoil 204A within the purification unit 200. The packed bed 206 can forcefluids to take complicated, torturous paths as the fluids flow throughthe purification unit 200, thereby creating increased surface area forinteraction (that is, contact) between different phases (for example,liquid sulfur and vapor SO₂) that exist within the purification unit200. The height of the packed bed 206 can depend on a calculated orpredicted rate of SO₂ emissions from the tank 110. Sulfur can be carriedby (that is, flow with) the vapor flow from the tank 110 through thevent line 112 to the purification unit 200. The packed bed 206 can causethe sulfur to be filtered or fall out of the vapor flowing through thepurification unit 200, and the sulfur can collect at the bottom of thepurification unit 200 and subsequently drained out using drain 224.

The absorber section 210 can be located above the tray 208, near the topof the purification unit 200. The absorber section 210 can remove atleast a portion of the SO₂ and H₂S from the vapor stream. FIG. 3 shows across-sectional view of an example absorber section 210. The absorbersection 210 can include a bed of catalyst, for example, a bed of ceramiccatalyst. As shown in FIG. 3 , the ceramic catalyst can have a honeycombstructure and can filter SO₂ and H₂S and absorb heat. As vapor flowsthrough the absorber section 210, SO₂ and H₂S in the vapor can beremoved. In some cases, the vapor carries some sulfur. Referring back toFIG. 2 , the tray 208 above the packed bed 206 can collect sulfur (thatis, liquid sulfur can accumulate on the tray 208), and the accumulatedsulfur can be drained out using drain 214. As one example, the drains214 and 224 can discharge fluid to a sulfur pit. The drains 214 and 224can (like the vent line 112 shown in FIG. 1 ) include heat tracing, suchas electric tracing, steam tracing, or a combination of both to maintaina temperature within the drains (214, 224) that is sufficiently high toprevent solidification of sulfur. For example, the heat tracing canmaintain a temperature of approximately 180° C. within the drains 214and 224.

The inlet 202 can allow fluid (for example, vapor from the tank 110flowing through the vent line 112) to enter the purification unit 200.The inlet 202 can be located near the bottom of the purification unit200. Vapor can travel up past the steam coil 204A, through the packedbed 206, past the tray 208, past the steam coil 204B, through theabsorber section 210, and then exit through the outlet 212 (located atthe top of the purification unit 200) with reduced SO₂ content.Therefore, the effluent from the purification unit 200 can be consideredto be cleaner than the vapor entering the purification unit 200 (thatis, the vapor from the tank 110). The manhole 222 can be opened, so thatan operator can enter the purification unit 200, for example, to performmaintenance on the various sections of the purification unit 200. ThePSV 226 can open to relieve fluid from the purification unit 200 in thecase that the pressure in the purification unit 200 rises above athreshold pressure (for example, the design pressure of the purificationunit 200) for any reason. Therefore, the PSV 226 provides overpressureprotection to the purification unit 200. Additional PSVs can beinstalled in the sulfur fume purification system 100 to provideoverpressure protection, such as on the tank 110 and on the vent line112.

While this specification contains many specific implementation details,these should not be construed as limitations on the scope of anyinvention or on the scope of what may be claimed, but rather asdescriptions of features that may be specific to particularimplementations of particular inventions. Certain features that aredescribed in this specification in the context of separateimplementations can also be implemented, in combination, in a singleimplementation. Conversely, various features that are described in thecontext of a single implementation can also be implemented in multipleimplementations, separately, or in any suitable sub-combination.Moreover, although previously described features may be described asacting in certain combinations and even initially claimed as such, oneor more features from a claimed combination can, in some cases, beexcised from the combination, and the claimed combination may bedirected to a sub-combination or variation of a sub-combination.

Particular implementations of the subject matter have been described.Other implementations, alterations, and permutations of the describedimplementations are within the scope of the following claims as will beapparent to those skilled in the art. While operations are depicted inthe drawings or claims in a particular order, this should not beunderstood as requiring that such operations be performed in theparticular order shown or in sequential order, or that all illustratedoperations be performed (some operations may be considered optional), toachieve desirable results.

Accordingly, the previously described example implementations do notdefine or constrain this disclosure. Other changes, substitutions, andalterations are also possible without departing from the spirit andscope of this disclosure.

The invention claimed is:
 1. A purification unit configured to process avapor stream comprising sulfur dioxide, the purification unitcomprising: a steam coil configured to circulate steam and provide asource of heat to a vapor stream comprising sulfur dioxide, the vaporstream received through an inlet of the purification unit; a packed bed;a tray above the packed bed, the tray configured to accumulate sulfur;an absorber section above the tray, the absorber section configured toremove at least a portion of the sulfur dioxide from the vapor stream;and an outlet at or near a top of the purification unit, the outletconfigured to allow an effluent with a lower sulfur dioxide content thanthe vapor stream to exit the purification unit.
 2. The purification unitof claim 1, wherein the packed bed comprises structured packing, randompacking, or combinations thereof.
 3. The purification unit of claim 1,wherein the packed bed comprises a wire mesh and rings, each ringcomprising a length and a diameter, the length being approximately equalto the diameter.
 4. The purification unit of claim 1, wherein theabsorber section comprises a bed of catalyst having a honeycombstructure.
 5. The purification unit of claim 1, wherein the steam coilis configured to maintain a temperature within the purification unit atapproximately 180 degrees Celsius (° C.).
 6. The purification unit ofclaim 1, wherein the purification unit further comprises a sulfur drainconfigured to allow accumulated sulfur flow from the tray to exit thepurification unit.
 7. The purification unit of claim 6, furthercomprising electric tracing, steam tracing, or combinations thereof tomaintain in the vent line and the sulfur drain, a sufficiently hightemperature to prevent solidification of sulfur.
 8. The purificationunit of claim 1, further comprising a manhole attached to a side of thepurification unit, the manhole configured to be opened to allow anoperator to enter the purification unit.
 9. The purification unit ofclaim 1, further comprising a pressure safety valve configured to opento relieve fluid from the purification unit in response to pressure inthe purification unit rising above a threshold pressure.
 10. Thepurification unit of claim 1, wherein the steam coil is a first steamcoil, wherein the purification unit comprises a plurality of steam coilsincluding the first steam coil.
 11. The purification unit of claim 10,wherein the first steam coil is located near a bottom of thepurification unit and the second steam coil is located near a middle ofthe purification unit.
 12. The purification unit of claim 10, whereinthe plurality of steam coils maintain a temperature within thepurification unit that is sufficiently high to prevent solidification ofsulfur.
 13. The purification unit of claim 1, further comprising steamjacketing to maintain temperature and prevent heat loss.